1. Field of the Invention
The present invention relates to a flat-shaped battery that is formed in a flat shape such as a coin shape or a button shape as a whole and is to be used in electronic equipment such as a wristwatch.
2. Description of Related Art
JP 50(1975)-12533 A and JP 59(1984)-166359 U disclose a flat-shaped battery configured so that a positive electrode can for housing a power generation element is used as an outer can and a sealing plate that seals an opening provided at the upper end of the positive electrode can by being fixed to the inner edge of the opening together with a gasket by caulking is used as a negative electrode can. In JP 50(1975)-12533 A and JP 59(1984)-166359 U, a gold plating layer is formed on the outer surface of a stainless steel as a base composing the negative electrode can.
When the outer surface of a stainless steel base is covered with the gold plating layer as disclosed in JP 50 (1975)-12533 A and JP 59 (1984)-166359 U, the contact resistance between the base and a connection terminal of electronic equipment or the like decreases as compared with the case where the stainless steel is in direct contact with the connection terminal. Thus, it is possible to obtain stable discharging characteristics. Furthermore, gold is less prone to be oxidized, so that an increase in contact resistance on the outer surface of the battery can hardly occur even when the battery is used for a long time. Thus, it is possible to obtain excellent discharging characteristics over a long time.
In general, a steel plate of iron or an iron alloy such as stainless steel composing the base of the outer can is a rolled steel plate formed by rolling, and a bright annealed finish (a specular gloss finish) is given to a surface of such a rolled steel plate so as to gloss and smooth out the surface. However, even after the above-described surface treatment, the surface of the base of the outer can still has recesses and projections of about a few micrometers in dimension formed during the processing of the base. These recesses and projections are about 5 μm in dimension when the rolled steel plate is a stainless steel plate and are about 8 μm in dimension when the rolled steel plate is an iron steel plate. Thus, when the thickness of the gold plating layer is small, it is difficult to form the plating layer uniformly on the outer surface of the outer can because the recesses and projections on the base exist on the surface of the outer can. Thus, portions without the gold plating layer (so-called pinholes) are formed easily. In the case where the pinholes are formed as described above, a decrease in the contact resistance cannot be obtained when the contact portion between the outer can and electronic equipment is a portion where the pinhole is present. In particular, when the base is a material that is corroded easily (such as iron), the pinholes expedite the corrosion of the outer can.
One remedy against this problem is to make the gold plating layer thick enough to cover the recesses and projections on the base so as to prevent the pinholes from being formed in the gold plating layer. However, since the thickness of the gold plating layer needs to be greater than a vertical interval (an interval between the crest and the trough) of the recesses and projections, the cost of the battery increases considerably. Moreover, gold is soft and achieves only low adhesion to a stainless steel plate. Thus, when the gold plating layer is formed so as to have a thickness that can cover the vertical interval of the recesses and projections as described above, the plating layer formed nonuniformly is liable to be damaged so that, at the time of a caulking process or the like during the production of the battery, the gold plating layer may adhere to the inner surface of a die to be detached easily. This brings about another problem that the thus-formed gold plating layer may be detached from the outer can.